Overheating 360 new engine
Want to make sure you are getting all the air out of the cooling system upon filling it up. The small blocks are difficult in this area.
If you are not getting a full 4 gallons of coolant into your system when first filling it, then there is air still in there creating hot spots that won't cool.
Here is a simple mod with the thermostat that lets the air out as you are filling.